Bradykinin B2-receptor-mediated positive chronotropic effect of bradykinin in isolated rat atria.

The positive chronotropic effect of bradykinin was investigated in isolated spontaneously beating atria of the rat. Cumulative additions of bradykinin (0.3-100 nM) caused a concentration-dependent increase in the beating rate of the atria by maximally 35+/-4 beats/min, approximately 25% of the 1 microM isoprenaline-induced maximal responses. In contrast, the active metabolite of bradykinin and selective bradykinin B1-receptor agonist, Des-Arg9-bradykinin, did not influence the spontaneous frequency of beating. Propranolol (1 microM) combined with prazosin (1 microM) did not affect the positive chronotropic effect of bradykinin. A selective bradykinin B2-receptor antagonist, Hoe 140, concentration-dependently shifted the response curves for bradykinin to the right, whereas the bradykinin B1-receptor antagonist, Lys-[Leu8]Des-Arg9-bradykinin had no effect. The tachycardic responses to bradykinin were potentiated by ramipril, an angiotensin-converting enzyme/kininase II inhibitor, but not affected by Nomega-nitro-L-arginine methyl ester hydrochloride, a nitric oxide synthesis inhibitor. Indomethacin and meclofenamate, two cyclooxygenase inhibitors, abolished the bradykinin-induced chronotropic effect. These results indicate that exogenous bradykinin induces a positive chronotropic effect that occurs independent of adrenoceptors. The bradykinin-induced chronotropic effect is mediated by bradykinin B2 receptors, whereas B1 receptors do not play a role in mediating this effect. Prostaglandins but not nitric oxide appear to be involved in bradykinin-induced positive chronotropic effect.